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An inductor doubtfully will be in a position to be of harm, however a solenoid with 1A+ running through it (and of course a lot of it stored in the magnetic field) can certainly harm the circuit, if not somebody touching it at one point.

From what I would imagine, the stored current will be converted back from magnetic flux and then be sent in to the circuit again, however, is there any harm in somebody happening to touch a (50mA->1A) solenoid when continuously on? When the circuit is accidentally ungrounded (not 100% my worry) or turned off?

The input voltage should always (hopefully) be under nine volts, and I will be using it as a small electromagnet maybe for experimenting, however if the release of current causes a large potential that is one of the concerns although I do have quite a lack of understanding.

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At 9V I wouldn't worry about touching the solenoid. After all, you can touch other parts of the circuit that are at 9V.

The only issue with a solenoid is that it can for a short period of time make a high voltage if its current is shut off suddenly. For example, hold the wires of a solenoid accross a 9V source, then watch what happens when you release one of the wires. You'll see a spark. This is indication of temporary high voltage. If you were touching the two wires at the time you'd feel this spark. But, its energy is limited and duration short, so other than being unpleasant won't do anything bad unless maybe you somehow manage to get the current running thru your heart or your brain.

The same temporary high voltage you can feel can also damage your circuit if you don't deal with the stored energy in the solenoid properly. The simplest way to do this is to put a diode in reverse accross the solenoid:

In this case transistor Q1 is just a example of something switching the solonoid on and off. L1 is the solenoid coil, which is just a inductor with significant series resistance from the point of view of the driving circuit. D1 is reverse biased and therefore does nothing when the solenoid is on. However, when the solenoid is switched off it provides a path for the current to flow until it decays by disspating against the inherent resistance of the solenoid and the voltage drop accross D1. The highest voltage ever in this circuit is 9V plus the forward drop accross D1. This circuit would be safe to touch with your hands like any other 9V circuit.

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  • \$\begingroup\$ One problem with the above circuit is that adding the diode may make release of the solenoid "spongy". If the solenoid needs to release crisply, one should add a resistor in series with the diode. To calculate the resistor value, compute the maximum current in the coil, and compute the maximum acceptable voltage across it (the transistor's maximum safe collector voltage, minus 9.7 volts), and size the resistor appropriately. The higher the value of the resistor, the more clean the solenoid release. \$\endgroup\$ – supercat Oct 31 '11 at 15:24
  • \$\begingroup\$ Thank you both of you, I appreciate the advise. It is always better to ask, and an excuse to show off safety portion of the circuit rather than just its results. \$\endgroup\$ – Kim N. Oct 31 '11 at 19:20
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Please elaborate what you mean by your statement that a solenoid with 1A current can 'harm' a circuit.

A properly designed circuit should have sufficient susceptibility margin to prevent self-disturbance from inductor switching. Also, any inductor that can have interrupted current (switching or otherwise) really should be clamped by an anti-kickback diode to limit the potentially high kickback voltage.

The only physical harm that I could imagine coming from a well-designed inductor circuit (i.e. one with no exposed conductive surfaces) is a burn, if the device temperature is too hot (usually greater than 50C) to safely make contact with.

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If the conductors and terminals are insulated, which they should be, there will not be any electrical hazard.

The main hazard from solenoids is mechanical. An exposed solenoid could trap fingers or other extremities in them causing the damage and the pain.

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